Directional drilling is the process of directing a borehole along a defined trajectory. Deviation control during drilling is the process of keeping the borehole trajectory contained within specified limits, e.g., limits on the inclination angle or distance from the defined trajectory. Both have become important to developers of hydrocarbon resources.
Every bottom-hole assembly (BHA) drilling a deviated borehole rests on the low side of the borehole, thereby experiencing a reactive force that causes the BHA to tend upward (increase borehole inclination due to a fulcrum effect), tend downward (decrease borehole inclination due to a pendular effect), or tend neutral (maintain inclination). Even for the same BHA, the directional tendencies may change due to formation effects, bit wear, inclination angle, and parameters that affect stiffness such as rotational speed, vibration, weight-on-bit (WOB), and wash-outs. Parameters that can be employed to intentionally affect directional tendency include the number, placement, and gauge of stabilizers, the bend angles associated with the steering mechanism, the distance of the bends from the bit, rotational speed, WOB, and rate-of-penetration (ROP).
Various drillstring steering mechanisms exist to provide directional drilling: whipstocks, mud motors with bent-housings, jetting bits, adjustable gauge stabilizers, and rotary steering systems (RSS). These techniques each employ side force, bit tilt angle, or some combination thereof, to steer the drillstring's forward and rotary motion. However, the resulting borehole's actual curvature is not determined by these parameters alone, and it is often difficult to predict the location of the bit during drilling. Such difficulty necessitates slow drilling, frequent survey measurements, and in many cases, frequent trips of the drillstring to the surface to adjust the directional tendency of the steering assembly. Such necessity produces undesirably undulatory and tortuous wellbores and the many problems associated therewith.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.